Automatically generate screenshots for all examples (#2379)

examples/hello_world/Cargo.toml

@@ -9,5 +9,7 @@ publish = false
 
 
 [dependencies]
-eframe = { path = "../../crates/eframe" }
+eframe = { path = "../../crates/eframe", features = [
+    "__screenshot", # __screenshot is so we can dump a ascreenshot using EFRAME_SCREENSHOT_TO
+] }
 tracing-subscriber = "0.3"

examples/hello_world/README.md

@@ -1,3 +1,7 @@
+Example showing some UI controls like `Label`, `TextEdit`, `Slider`, `Button`.
+
 ```sh
 cargo run -p hello_world
 ```
+
+![](screenshot.png)

examples/hello_world/src/main.rs

@@ -6,7 +6,10 @@ fn main() {
     // Log to stdout (if you run with `RUST_LOG=debug`).
     tracing_subscriber::fmt::init();
 
-    let options = eframe::NativeOptions::default();
+    let options = eframe::NativeOptions {
+        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        ..Default::default()
+    };
     eframe::run_native(
         "My egui App",
         options,